Audio equipment using electronically-operated tone controls may use the so-called Voss tone control circuit shown schematically in FIGS. 1A through 1C.
In the Voss tone control circuit, a frequency cut characteristic is obtained by connecting the input signal to the non-inverting input 48 of the amplifier 10 via the frequency-selective network 12 and to the inverting input 50 via the input resistor 14, as shown in FIG. 1B. The amount of cut is selected by connecting the non-inverting input to a different point on the potentiometer 16 by selecting one of the plural electronically-controlled switches 18 through 25 (only the switches 18, 24, and 25 are shown in this example to simplify the drawing). The number of switches required depends on the desired fineness of the increments in the maximum amount of cut. Whether the circuit provides bass cut or a treble cut characteristic is determined by the frequency characteristics of the frequency-selective network 12, which, in the Voss circuit, is a band-pass filter.
A frequency boost characteristic is obtained by connecting the same frequency-selective network 12 between the output 52 and the non-inverting input 48 of the amplifier 10, as shown in FIG. 1A. The amount of boost is selected by connecting the non-inverting input to a different point on the potentiometer 16 by selecting one of the plural electronically-controlled switches 18 through 25.
Common electronically-controlled switches 18 through 25, and a common frequency-selective network 12 can be used to provide both boost and cut characteristics by adding the boost-cut changeover switch 32, as shown in FIG. 1C. This arrangement reduces circuit complexity, the external component count, and the number of package pins required.
The known Voss circuit uses the frequency selective network 12 shown in FIG. 1D, consisting of the resistor 40, the potentiometer 16, and two capacitors 44 and 46. In the Voss circuit, equal values (e.g., 68 nF in a bass control circuit) are used for the capacitors 44 and 46, and the values of the resistor 40 and the potentiometer 16 are selected to give the required characteristics. This results in the resistor 40 having a value of about 14k, which is less than one tenth of that of the potentiometer 16 (160k) . Thus, the value of the resistor 40 is quite low, with the result that the resistor 40 imposes a considerable load on the boost-cut switch 32. This, in turn, requires that this switch must have a large area to provide an acceptably low level of distortion.
Also, in the known Voss circuit, the input current for the non-inverting input 48 of the amplifier 10 flows through the part of the potentiometer 16. Thus, the source resistance seen by the non-inverting input can be as high as about thirteen times that seen by the inverting input 50 (provided by the parallel combination of resistors 14 and 30, which are about 20k each). This causes a DC offset on the output of the amplifier 10.
Moreover, the source resistance seen by the non-inverting input varies, depending on which one of the electronic switches 18 through 25 is ON. When the switch 18 is ON (maximum boost or cut), the source resistance seen by the non-inverting input is as high as about 130k in the known circuit. When the switch 25 is ON, the source resistance seen by the non-inverting input is as low as the ON resistance of the electronic switch 25. The source resistance provided by the potentiometer 16 changing as the switches 18-25 are operated changes the DC level at the output of the amplifier 10, resulting in potentially audible switching clicks when the amount of boost or cut is changed.
Since the ratio between the values of the resistor 40 and the potentiometer 16 is set by the desired frequency selection characteristic of the frequency-selective network 12, increasing the value of the resistor 40 to reduce the load on the boost-cut switch 32 in the known circuit will increase the value of the potentiometer 16, which will worsen the offset and switching click problem. Alternatively, decreasing the value of the potentiometer 16 to reduce the offset and switching click problem in the known circuit requires that the value of the resistor 40 also be reduced, which results in increased distortion from the boost-cut switch.
Increasing the value of the input resistor 14 and the feedback resistor 30 would reduce the amount of source resistance imbalance, and, hence, the offset problem. However, increasing the value of each of these resistors to provide a DC source resistance in the range of average source resistance seen by the non-inverting input 48 could result in an unacceptably high output noise level.